MediaProcessing.asCIImageSequence should produce an array of downsampled frames

[davidkoski]

MediaProcessing.asCIImageSequence should take a visitor that can apply processing to frames and downsample (requires rendering as CIImage is just a promise!)

Right now we do this:

            var videoFrameResult = await try MediaProcessing.asCIImageSequence(
                video, maxFrames: maxVideoFrames, targetFPS: targetVideoFPS)

            var processedFrames: [MLXArray] = []
            for frame in videoFrameResult.frames {
                let image =
                    frame
                    .toSRGB()
                    .resampled(
                        to: CGSize(width: fixedImageSize, height: fixedImageSize), method: .lanczos
                    )
                    .normalized(mean: config.imageMeanTuple, std: config.imageStdTuple)
                    .asMLXArray()
                processedFrames.append(image)
            }

if the video is 4k, we might have 64 very large frames that we then downsample rather than downsampling as we read.

[davidkoski]

See also #206 and #222